Before considering detail of particular embodiments of the invention it is convenient to note that when a turbine or kinetic energy converter (of any kind) is used in such a way that it is driven by a flow of water, the extraction of energy from the flow causes reduction in momentum of the passing water which in turn causes large reaction forces on the turbine which manifest themselves primarily as a thrust force acting in the direction of the flow (i.e. horizontally) and proportional to the numerical square of the mean velocity through the rotor.
This phenomenon is a consequence of the laws of physics, resulting from the transfer of momentum in the flowing water to the moving turbine components, and will occur regardless of the type of turbine rotor that is used. In general, the more powerful and efficient the turbine rotor, the greater the forces that need to be resisted, although under certain conditions, such as a so-called “run away” caused by loss of load, large thrust forces can be produced even when the turbine is not delivering much, or indeed any, usable power to the shaft of the turbine. This condition is of course a direct consequence of the fact that the forces needed to hold the rotor in position are the reaction to the forces transmitted to the turbine rotor in order to rotate it, which in turn give a measure of its efficacy for the generation of shaft power.
Moreover, in practice any such turbine is exposed to numerous cyclic loads caused by phenomena such as the effects of turbulence, passing waves, velocity shear in the water column (i.e. variation in velocity with depth) and vortex shedding, all of which will impose fluctuating fatigue loadings on the support structure. Such fluctuating loads need to be allowed for in providing adequate structural integrity. Therefore an essential requirement for any such turbine is that the rotor which extracts the energy must be held securely in position in the moving water column by a structure with adequate reserves of strength to resist the static and also the dynamic forces imposed on the rotor.
Provision of such a structure to which the present invention elates in practice complicated by a number of other general requirements, namely:—                the wake generated by the presence of the support structure in the water column shall not unduly interfere with the flow through the rotor or rotors (or it would reduce the efficiency of said rotor(s)). In fact the structure preferably needs to be configured such that its wake ideally completely misses the rotor(s).        the structure should also be as economical to manufacture as possible in order to minimise the costs of the system,        there needs to be some practical and cost-effective method for installing the supporting structure in a location with strong currents,        there needs to be some practical and cost-effective-method for installing the turbine rotor or rotors onto the structure and then for gaining access to the rotor or rotors and their associated power trains in order to maintain and when necessary to replace or repair them.        there needs to be some provision for the eventual removal for repairs, replacement or decommissioning of the structure        
Various more detailed considerations relate to the provision of a support structure for water current turbines include factors such as follows:—
Firstly, it should be noted that the flow in the water column at locations with high current velocities varies with depth such that the maximum velocity tends to be near to the surface. Conversely, the currents low in the water column, near the sea (or estuary or river) bed move much more slowly. Moreover, any uneven natural features in the sea, river or estuary bed will cause disruption of the flow near the seabed and extra turbulence; the more uneven and rougher the nature of the bed, the greater will be the thickness of a slower moving and turbulent lower layers.
Secondly, it should be noted that for efficient and reliable extraction of kinetic energy from water currents, using a turbine rotor of the kind proposed, it is desirable for water flow through said rotor to be as uniform as possible in velocity across the area swept by the rotor or rotors, to move as fast as possible and to have as little turbulence as possible. In other words it is desirable to have means to position the active rotor(s) in the fastest and most uniform and turbulence-free flows, avoiding the rotor cutting through any boundary layer or wakes caused by flow over an uneven sea, river or estuary bed. It is also essential to support any such rotor(s) with a structure capable of resisting for many years the most extreme static and dynamic forces that will be experienced.
Thirdly, an important further consideration is that any device immersed in the currents in a water column (whether in the sea, a river or an estuary) will need to be accessed occasionally for maintenance, repairs or replacement. Underwater operations in fast flowing currents, whether by human personnel wearing diving equipment or whether by Remotely Operated underwater Vehicles (ROVs), are extremely difficult or even impossible. As a result a fundamental problem to be addressed by this invention is the provision of means for accessing all components needing maintenance or repair, in particular the turbine rotor(s) and/or hydrofoils together with the mechanical drive train and generator which they activate, by making it possible to raise said items above the surface of the flowing water current so that access from a surface vessel is possible and no underwater intervention by divers or remotely operated submersible vehicles is needed.